Protein kinases represent one of the largest super-families of drug targets across all therapeutic areas. The central challenge in the development of kinase inhibitor drug candidates is in targeting the disregulated kinase while avoiding inhibition of non-disease related kinases containing closely related ATP binding pockets. Imatinib, the first clinically approved kinase inhibitor provided a remarkable example of a highly selective inhibitor of the translocation product Bcr-Abl (SEQ ID NO:1) (Capdeville et al., 2002, Nat Rev Drug Discov 1:493-502; Sawyers, 2002, Cancer Cell 1:13-15). Imatinib potently inhibits Bcr-Abl, the oncogene which drives chronic myelogenous leukaemia, but does not inhibit the cytoplasmic tyrosine kinase, c-Src (SEQ ID NO:2), despite the fact that the two kinases share almost completely identical amino acids lining the ATP binding pocket which Imatinib contacts (FIG. 1A; Schindler et al., 2000, Science 289:1938-1942; Seeliger et al., 2007, Structure 15:299-311). Significant medicinal chemistry, structural biology, and computational modelling efforts have focussed on understanding the differential selectivity of Imatinib for Bcr-Abl and c-Src.
The first insight into the basis for selectivity of Imatinib was revealed when Kuriyan and co-workers solved the Imatinib-Abl co-crystal structure (Nagar et al., 2002, Cancer Research 62:4236-4243; Schindler et al., 2000, Id.). This structure revealed a not-previously observed kinase conformation indicating that Imatinib binds Abl in a catalytically inactive conformation defined by a crank shaft-like displacement of the N-terminal region of the activation loop of the kinase effecting a dramatic change in the conformation of the Asp-Phe-Gly (DFG) triad. This conformational change has been subsequently observed in other protein kinase-drug co-crystal structures (Irk, Kit, Flt3, p38 Mapk and B-Raf; Griffith et al., 2004, Mol Cell 13:169-178; Hubbard et al., 1994, Nature 372:746-754; Mol et al., 2004, J Biol Chem 279:31655-31663; Pargellis et al., 2002, Nat Struct Biol 9:268-272; Wan et al., 2004, Cell 116:855-867) and has been termed the “type-II” or “DFG-out” conformation. ATP competitive inhibitors which bind to kinases in the active conformation are termed “type-I” or “DFG-in” binders; FIGS. 1B and C; Liu and Gray, 2006, Nat Chem Biol 2:358-364). The identification of an inactive conformation of Abl bound by the highly selective inhibitor Imatinib has guided many successful medicinal chemistry campaigns in search of selective kinase inhibitors (Angell et al., 2008, Bioorg Med Chem Lett 18:4433-4437; Cumming et al., 2004, Bioorg Med Chem Lett 14:5389-5394; Gill et al., 2005, J Med Chem 48:414-426; Heron et al., 2006, Bioorg Med Chem Lett 16:1320-1323; Okram et al., 2006, Chem Biol 13:779-786).
A wealth of data currently supports the view that the Imatinib bound conformation (DFG-out) of Abl is thermodynamically stable in complex with Imatinib, but that such conformations require energetically unfavourable interactions in c-Src complexes (Levinson et al., 2006, PLoS Biol 4:e144; Nagar et al., 2002, Id.; Seeliger et al., 2007, Id.; Vajpai et al., 2008, J Biol Chem 283:18292-18302). Imatinib has been crystallized in both its potent target Abl (Nagar et al., 2002, Id.; Schindler et al., 2000, Id.), as well as the poorly inhibited target, c-Src (Seeliger et al., 2007, Id.). Surprisingly, the Imatinib/co-crystal structures are virtually identical despite the significantly different affinities of Imatinib for the two protein kinases. Efforts to construct mutant forms of c-Src with the ability to be potently inhibited by Imatinib were only partially successful, which led Kuriyan and co-workers to suggest a distributed thermodynamic penalty for c-Src to adopt the DFG-out conformation (Seeliger et al., 2007, Id.). The importance of kinase conformational preference over precise amino acid identity is highlighted by studies with the Imatinib target receptor kinase, c-Kit (SEQ ID NO:3). Although c-Kit is more closely related to c-Src than Abl (SEQ ID NO:11) in the amino acids lining the ATP binding pocket, c-Kit is more potently inhibited by Imatinib (Deininger et al., 2005, Blood 105:2640-2653). Structural studies of c-Kit in the absence of ligand (ATP or Imatinib) show the kinase adopts the DFG-out conformation, suggesting the Imatinib bound conformation is stable and pre-formed in the absence of Imatinib, thereby explaining its Imatinib sensitivity (Mol et al., 2004, Id.)
Without wishing to be bound by any theory, it is widely held that the explanation of the discrepancy in affinity of Imatinib despite the close similarity in structure of the two drug-protein complexes is based on the relative propensity of the two kinases to adopt the relevant drug-bound (DFG-out/type II) conformation: Abl is predicted to prefer the DFG out conformation relative to c-Src, and since Imatinib binds to the type-II conformation of the kinase, its affinity is higher to Abl than to c-Src.